Would love to see some additional documented filters, actions, and other APIs for custom integration.
Wishlist
* An API for adding widgets to the designer so I could add a button for integrating content of my custom post type.
* A way of dynamically filtering email addresses out of a list when sending. If I have a newsletter of people who are interested in events, I don't want to keep pestering them to RSVP for an event they have already registered for. So filter registered attendees out of the main list when sending promotions. Send registrants different follow up messages. Currently, I think I'd have to create different lists for each event to make this work.
* I have something in my own email utility that grabs the YouTube thumbnail image of any post that includes a YouTube url. I'd like to see MailPoet do the same thing -- if no featured image is configured for a post that includes a YouTube video, display the YouTube thumbnail as the featured image for that post.
* A variation on the above would be for you to provide a filter developers could work in to alter the appearance of a blog preview or featured image (would me allow me to add that YouTube preview). I wouldn't necessarily want to alter how the post is displayed on the website; this would be specifically for display in a MailPoet email.